This study aims to understand the changes in age, total length, body weight, and muscle fat content in the Japanese eel Anguilla japonica during silvering, from yellow to silver eels, and their physiological significances. Eels were collected from the downstream of the Kaoping River of southern Taiwan from November 1998 through November 1999. The female eels were classified into three developmental stages based on external skin coloration and oocyte diameter, namely yellow, pre-silver and silver eel stages. The male eels were classified into yellow and silver eel stages only. The results showed that the occurrence of pre-silver females are found mainly in summer (Jun. - Aug.), while silver eels was found mainly in the winter and early spring (Nov. - Feb.), irrespective of sexes. The mean length and weight of the silver eels were 54.8 ± 1.8 cm and 269.8 ± 28.3 g in males, and were 63.1 ± 1.5 cm and 437.4 ± 41.8 g in females, respectively. The mean size of the silver female was significantly larger than that of males (p&lt;0.001). The mean age of the silver male and females were 5.9 ± 0.4 and 6.7 ± 0.3 years, respectively, and no significant difference was found between sexes (p&gt;0.05). The mean muscle fat contents in male and female silver eels were 17.0 ± 1.0 % and 17.6 ± 0.7 %, respectively, and no significant difference was found between sexes (p&gt;0.05). No significant differences of total length, body weight, age and muscle fat content were found between pre-silver and silver females (p&gt;0.05). During the growth phase of the yellow eels, the increases of total length, body weight, age and muscle fat content were positively correlated to each other (p&lt;0.05). These results indicated that the onset of silvering in the Japanese eels was season- related, and eels have to reach certain size, age, and muscle fat content for silvering to occur.